This study involves determining the physiological role of two proteins isolated from brain, a neuron specific enolase (NSE) and non-neuronal enolase (NNE). The former is a specific neuronal antigen and the latter a glial antigen. Studies are aimed at elucidating the physiological significance of these strictly segregated cellular isoenzymes as well as their utilization as molecular markers in probing the functional organization of brain. Developmental, immunocytochemical and functional studies have established that NNE and NSE represent the first example in neurobiology of strict glial and neuronal markers respectively. NSE is only present in highly differentiated neurons. NSE has also very recently been shown to be present in APUD (amine precursor reuptake and decarboxylation) cells, a finding that supports the contention that neurons and neuroendocrine cells are closely related.